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United States Patent |
5,318,723
|
Hashemi
|
June 7, 1994
|
Ceramic material and process for its production
Abstract
Conductive ceramic materials are provided which have resistivities at room
temperature of less than 10.sup.-3 .OMEGA..cm. These novel materials may
be made by forming a ceramic starting material comprising oxides of at
least two different metals, one of which is capable of existing in a +2
oxidation state and one of which is capable of existing in a +4 oxidation
state and exposing the ceramic starting material to reducing conditions.
Inventors:
|
Hashemi; Tooraj (Belmont, GB3)
|
Assignee:
|
Elmwood Sensors, Ltd. (GB2)
|
Appl. No.:
|
741513 |
Filed:
|
October 8, 1991 |
PCT Filed:
|
February 7, 1990
|
PCT NO:
|
PCT/GB90/00186
|
371 Date:
|
October 8, 1991
|
102(e) Date:
|
October 8, 1991
|
PCT PUB.NO.:
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WO90/09669 |
PCT PUB. Date:
|
August 23, 1990 |
Foreign Application Priority Data
Current U.S. Class: |
252/519.1; 252/519.13; 252/520.1; 501/134; 501/135 |
Intern'l Class: |
H01B 001/08 |
Field of Search: |
501/134,135
252/518,519,520,521
429/220,218
|
References Cited
U.S. Patent Documents
4222885 | Sep., 1980 | Hennings et al. | 501/136.
|
4244830 | Jan., 1981 | Hennings et al. | 501/138.
|
4284521 | Aug., 1981 | Payne et al. | 501/136.
|
4347167 | Aug., 1982 | Payne et al. | 501/138.
|
4425556 | Oct., 1984 | Hanke et al.
| |
5008163 | Apr., 1991 | Smith et al. | 252/518.
|
Foreign Patent Documents |
41860/72 | May., 1971 | AU.
| |
0040391 | May., 1981 | EP.
| |
2304032 | Jan., 1973 | DE.
| |
620437 | Nov., 1944 | GB.
| |
2008555 | Nov., 1978 | GB.
| |
Other References
"Ceramics as Electrical Materials" vol. 5, pp. 292, 294 & 299 Sprechsaal
107. Jahrgang-pp. 1057-1060, 1974.
Journal of the Electrochemical Society, Jul. 1987, pp. 1591-1594.
|
Primary Examiner: Group; Karl
Attorney, Agent or Firm: Birch, Stewart, Kolasch & Birch
Claims
I claim:
1. A conductive ceramic material which has a resistivity at ambient
temperature of less than 10.sup.-' .OMEGA..cm, comprising an ionic lattice
of metal ions and oxygen ions wherein at least a portion of said metal
ions are ions of a metal which exists in a plurality of valence states,
and wherein ions of at least two different metals A and B are present,
wherein at least some of said metal A exists in a +2 oxidation state,
wherein at least some of said metal B exists in a +4 oxidation state, and
wherein at least one of said two different metals A and B is present in
the form of ions having different oxidation states.
2. The conductive ceramic material according to claim 1, which has a
resistivity at ambient temperature of less than 10.sup.-4 .OMEGA..cm.
3. The conductive ceramic material according to claim 1 or claim 2, wherein
the presence of ions of different oxidation states is achieved by exposing
a ceramic starting material to reducing conditions.
4. The conductive ceramic material according to claim 1, wherein the
presence of ions of different oxidation states is achieved by exposing a
ceramic starting material to a reducing atmosphere.
5. The conductive ceramic material according to claim 1 or claim 2, wherein
the presence of ions of different oxidation states is achieved by
incorporating one or more electron-donating dopants in said ceramic
material.
6. The conductive ceramic material according to claim 5, wherein said
dopants comprise trivalent metal oxides selected from the group consisting
of Sb.sub.2 O.sub.3 and Bi.sub.2 O.sub.3.
7. The conductive ceramic material according to claim 1, comprising metal
oxides of said first metal A and said second metal B, said first metal A
being a metal selected from group Ib, IIa, IIb and VIII of the periodic
Table, and said second metal B being a metal selected from group IVa and
IVb of the Periodic Table and the lanthanides.
8. The conductive ceramic material according to claim 7, wherein said first
metal A is a member selected from the group consisting of Cu, Zn, Cd, Fe,
Sr and Ca.
9. The conductive ceramic material according to claim 7, wherein said
second metal is a member selected from the group consisting of Ti, Ge, Sn,
Pb, Ce and Pr.
10. The conductive ceramic material according to claim 7, wherein said
first metal B a member is copper and said second metal B is tin.
11. The conductive ceramic material according to claim 1, having the
empirical formula A.sub.2-x BO.sub.4-x-y, wherein x is from 0 to 1, and y
is less than 0.1.
12. A method of producing a ceramic material having a resistivity at
ambient temperature of less than 10.sup.-3 .OMEGA..cm, which comprises
forming a ceramic starting material comprising oxides of at least two
different metals A and B, wherein at least some of said metal A exists in
a +2 oxidation state, wherein at least some of said metal B exists in a +4
oxidation state, and wherein at least one of said two different metals A
and B is present in the form of ions having different oxidation states,
and exposing said ceramic starting material to reducing conditions.
13. The method according to claim 12, wherein said ceramic starting
material is exposed to a reducing atmosphere at an elevated temperature.
14. The method according to claim 13, wherein said ceramic starting
material is exposed to gaseous hydrogen.
15. the method according to claim 12, wherein said ceramic starting
material is subjected to electrochemical polarisation.
16. The method according to any one of claims 12 to 15, wherein said
ceramic starting material has the formula A.sub.2-x BO.sub.4-x.+-.y,
wherein x is from 0 to 1, and y is less than 0.1.
17. The method according to any one of claims 12 to 15, wherein said metal
known to exist A state is selected from group Ib, IIa, IIb and VIII of the
Periodic Table.
18. The method according to claim 17, wherein said metal A state is
selected from the group consisting of Cu, Zn, Cd, Fe, Sr and Ca.
19. The method according to any one of claims 12 to 15, wherein said metal
B is selected from group IVa and IVb of the Periodic Table, or is a
lanthanide.
20. The method according to claim 19, wherein said metal B is a member
selected from the group consisting of Ti, Ge, Sn, Pb, Ce and Pr.
21. The method according to any of claims 12 to 15, wherein said ceramic
starting material is exposed to hydrogen which is diluted with an inert
gas.
22. The method according to claim 21, wherein said ceramic starting
material is exposed to a reducing gas which comprises 10 to 60% by volume
hydrogen, with the balance being provided as an inert gas.
23. The method according to any of claims 12 to 15, wherein the exposure to
reducing conditions carried out at a temperature greater than 250.degree.
C.
24. The method according to claim 23, wherein said temperature is in the
range 300.degree. to 500.degree. C.
25. The conductive ceramic material according to claim 3, wherein the
presence of ions of different oxidation states is achieved by exposing a
ceramic starting material to an electrolyte, and subjecting said material
to electrochemical polarisation.
26. A conductive ceramic material which has a resistivity at ambient
temperature of less than 10.sup.-3 .OMEGA..cm, comprising an ionic lattice
of metal ions and oxygen ions wherein at least a portion of said metal
ions are ions of a metal which exists in a plurality of valence states,
and wherein ions of at least two different metals A and B are present,
wherein at least some of said metal A exists in a +2 oxidation state,
wherein at least some of said metal B exists in a +4 oxidation state, and
wherein at least one of said two different metals A and B is present in
the form of ions having different oxidation states,
wherein said metal A is one selected from group Ib, IIa, IIb, and VIII of
the Periodic Table, and
wherein said metal B is one selected from group IVa and IVb of the Periodic
Table, and the lanthanides.
27. The conductive ceramic material according to claim 26, wherein said
metal A is a member selected from the group consisting of Cu, Zn, Cd, Fe,
Sr, and Ca.
28. The conductive ceramic material according to claim 26, wherein said
metal B is a member selected from the group consisting of Ti, Ge, Sn, Pb,
Ce, and Pr.
29. The conductive ceramic material according to claim 26, wherein said
metal A is copper, and wherein said metal B is tin.
30. The conductive ceramic material according to claim 26, having the
empirical formula A.sub.2-x BO.sub.4-x-y, wherein said metal A is selected
from group Ib, IIa, IIb, and VIII of the Periodic Table, said metal B is
selected from group IVa and IVb of the Periodic Table, and the
lanthanides, x is from 0 to 1, and y is less than 0.1.
31. A method of producing a ceramic material having a resistivity at
ambient temperature of less than 10.sup.-3 .OMEGA..cm, which comprises
forming a ceramic starting material comprising oxides of at least two
different metals A and B, wherein at least some of said metal A exists in
a +2 oxidation state, wherein at least some of said metal B exists in a +4
oxidation state, and wherein at least one of said two different metals A
and B is present in the form of ions having different oxidation states,
and exposing said ceramic starting material to reducing conditions,
wherein said metal A is one selected from group Ib, IIa, IIb, and VIII of
the Periodic Table, and
wherein said metal B is one selected from group IVa and IVb of the Periodic
Table, and the lanthanides.
32. The method according to claim 31, wherein said ceramic starting
material has the formula A.sub.2-x BO.sub.4-x-y, wherein x is from 0 to 1,
and y is less than 0.1.
33. The method according to claim 32, wherein said metal A is a member
selected from the group consisting of Cu, Zn, Cd, Fe, Sr, and Ca.
34. The method according to claim 32, wherein said metal B B is a member
selected from the group consisting of Ti, Ge, Sn, Pb, Ce, and Pr.
Description
BACKGROUND OF THE INVENTION
The present invention relates to ceramic materials having low resistivities
and processes for their production.
DESCRIPTION OF RELATED ART
Electrically conductive ceramic materials are used in many fields where
advantage is taken of their characteristic electrical properties. Thus,
for examples, conductive ceramics are used in the manufacture of
electrical components such as resistance elements, capacitors and
semi-conductor devices. Generally, however, ceramic materials have
relatively high resistivities, typically greater than 10.sup.-1
.OMEGA..cm. Thus, for example, evaporated films of stannic oxide
(SnO.sub.2) are widely used as electrical resistance elements, but
resistivities less than 10.sup.-1 .OMEGA..cm are difficult to obtain.
Resistances of 1 to 10 .OMEGA..cm can be observed in freshly prepared
samples of cadmium oxide (CdO) but this material is relatively unstable.
Resistivities in the range 10.sup.-1 to 10.sup.-3 .OMEGA..cm have been
observed in sintered samples of cadmium stannate (see T. Hashimi et al.,
J. Electrochem. Soc. 134 (1987) pp. 1591-1594) where the use of such
relatively low resistivity ceramic materials in the manufacture of
electrodes for secondary electrochemical cells is described. However
hitherto, the production of ceramic materials having resistivities
substantially less than 10.sup. -3 .OMEGA..cm has never been described.
The production of such low resistivity ceramic materials would be highly
desirable, as it would enable such materials to be used in place of
metals, which typically have resistivities in the range 10.sup.-3 to
10.sup.-7 .OMEGA..cm. Thus, for example, low resistivity ceramics could be
used in corrosive environments, where hitherto expensive noble metals such
as platinum and gold have hitherto been used.
SUMMARY OF THE INVENTION
We have now developed a highly conductive ceramic material which has a
resistivity at ambient temperature of less than 10.sup.-3 .OMEGA..cm.
The ceramic materials according to the invention would normally not have
resistivities lower than 10.sup.-7 .OMEGA..cm. Samples have, however, been
produced with resistivities less than 10.sup.-4 .OMEGA..cm.
The conductive ceramic materials according to the invention generally
comprise an ionic lattice of metal ions and oxygen ions wherein at least a
portion of the metal ions are ions of a metal capable of existing in a
plurality of valence states, and wherein ions of at least two different
metals are present, one of which is capable of existing in a +2 oxidation
state and one of which is capable of existing in a +4 oxidation state and
wherein at least one of said two different metals is present in the form
of ions having different oxidation states.
The presence of ions of different oxidation states is achieved according to
one embodiment of the invention by exposing a ceramic starting material to
reducing conditions, for example by exposing it to a reducing atmosphere.
Alternatively ions of one or more of the metals may be maintained in
different oxidation states by incorporating one or more dopants in the
ceramic material. Examples include electron-donating dopants, such as
trivalent metal oxides, e.g., Sb.sub.2 O.sub.3 or Bi.sub.2 O.sub.3.
The invention also provides a method of producing a ceramic material having
a resistivity at ambient temperature of less than 10.sup.-3 .OMEGA..cm,
which comprises forming a ceramic starting material comprising at least
two different metals in the form of oxides, one of which is capable of
existing in a +2 oxidation state and one of which is capable of existing
in a +4 oxidation state, said ceramic starting material having a
resistivity greater than 10.sup.-3 .OMEGA..cm and exposing the ceramic
starting material to reducing conditions, generally at an elevated
temperature. Preferably the ceramic starting material is exposed to a
reducing atmosphere, for example gaseous hydrogen although other methods
may be employed, for example electrochemical polarization.
The selection of the metal oxide components of the ceramic starting
material and the processing conditions are major factors contributing to
the low resistivity of the resulting ceramic material. Oxides of
individual metals may be mixed and subjected to conventional procedures
for forming ceramics such as compacting, and sintering. Alternatively,
mixed oxides or salts comprising at least two metals may be subjected to
such procedures whereby the starting materials are converted to metal
oxide form during processing.
Thus the metal capable of existing in a +2 oxidation state (hereinafter
referred to as "the first metal") is preferably selected from groups Ib,
IIa, IIb and VIII of the periodic table, most preferably from Cu, Zn, Cd,
Fe, Sr and Ca.
The metal capable of existing in a +4 oxidation state (hereinafter referred
to as "the second metal") is preferably selected from group IVa and IVb of
the periodic table or is a lanthanide. Most preferably, the metal is Ti,
Ge, Sn, Pb, Ce or Pr.
Examples of empirical formulae which may be written for ceramic materials
formed from metals existing in the +2 and +4 oxidation states include
ABO.sub.3 and A.sub.2 BO.sub.4 wherein A and B represent the first and
second metals respectively. Intermediate compositions may be formed by
varying the proportions of starting materials and generally the ceramic
starting material used in the method of the invention may have an
intermediate composition which may be described by the general formula
A.sub.2-x BO.sub.4-x where x is in the range 0 to 1. Often, the ceramic
starting material may, on analysis, be found to have a non-stoichiometric
composition, which may be represented by the formula A.sub.2-x
BO.sub.4-x.+-.y where y is less than 0.1. After having been exposed to
reducing conditions, the ceramic starting material will become oxygen
deficient and the resulting highly conductive ceramic material may be
represented by the formula A.sub.2-x BO.sub.4-x-y. Other ions,
particularly monovalent species will generally be present to provide the
necessary charge balance.
Ceramic starting materials may also be used in accordance with the
invention which include an excess of one or the other of the
aforementioned metal oxides and/or other metal oxides.
The ceramic starting material used in the method of the invention may be
produced by conventional techniques, for example by the co-comminution of
metal oxides or by the co-precipitation of insoluble compounds of the
metals, e.g. oxides, hydroxides or carbonates followed, if necessary, by
calcining. Preferably, however, oxides of the first and second metals are
co-comminuted by, for example ball-milling, followed by compacting so as
to form a "green" pellet under high pressure. The resulting pellet may
then be sintered, preferably in an oxidizing atmosphere, for example air,
so as to form a solid, ceramic body. The ceramic body so-formed will
generally be found to have a resistivity typical of conventional ceramic
materials, i.e. greater than 10.sup.-1 .OMEGA..cm, although when certain
combinations of metal oxides are used, for example cadmium oxide and tin
oxide as described in the procedure of Hashimi et al. supra, resistivities
as low as 10.sup.-3 .OMEGA..cm may be observed.
In order to form a ceramic material having a resistivity less than
10.sup.-3 .OMEGA..cm in accordance with the invention, it is generally
necessary to expose the ceramic starting material to reducing conditions.
Thus, for example, the ceramic starting material may be exposed to a
reducing atmosphere, or it may be reduced electrochemically by exposing
the material to an electrolyte and subjecting the material to
electrochemical polarisation.
A preferred method is to subject the ceramic starting material to a heat
treatment or annealing step under a reducing atmosphere. As indicated
above, such treatment may be unnecessary if the ceramic starting material
contains dopants capable of introducing electron-deficient centres into
the lattice. Preferably, the annealing step is carried out in the presence
of hydrogen which optionally may be dilluted with an inert gas such as,
for example, nitrogen. Most preferably the reducing gas comprises 10 to
60% by volume hydrogen with the balance being provided as an inert gas.
The annealing step is preferably carried out at a temperature greater than
250.degree. C., most preferably in the range 300.degree. to 500.degree. C.
Prior to carrying out the step of exposing the ceramic starting material to
reducing conditions in accordance with the invention, the starting
material may be subjected to standard procedures for controlling the
density, porosity and shape. Thus, for example, standard techniques used
in ceramic processing may be employed such as, for example, hot isostatic
pressing, extrusion, compaction and molding.
The resulting low resistivity ceramic material formed in accordance with
the invention may be subjected to a grinding step in order to form a
highly conductive ceramic powder. However if the material is required in
powder form, a preferred procedure is to prepare the ceramic starting
material in powder form using a procedure such as, for example flame
spraying or spray pyrolysis heating and then if necessary, exposing the
so-formed ceramic powder to a reducing atmosphere at an elevated
temperature in the manner described above.
Ceramic materials may be formed by the process of the invention which have
resistivities which are comparable to those of metals. Typically,
resistivities of 10.sup.-4 .OMEGA..cm and lower are achievable. Specimens
have been produced according to the invention which have resistivities
lower than 10.sup.-5 .OMEGA..cm, e.g. around 10.sup.-6 .OMEGA..cm, which
approach the resistivities of noble metals. Further, the low resistivity
ceramic materials produced according to the invention are relatively
stable and test specimens have been produced which continue to exhibit
resistivities as low as 10.sup.-5 .OMEGA..cm after being kept for 2 weeks
at room temperature.
The production of ceramic materials having a resistivity less than
10.sup.-3 .OMEGA..cm in accordance with the invention is entirely
unexpected and the theoretical basis for the observed low resistivities
has not yet been determined. Possibly, the observed conductivity is a
result of a mechanism dependent upon the existence of oxygen vacancies in
the crystal lattice. The observed conductivity may stem from a mechanism
which is analogous to the mechanisms for electrical conductance suggested
for semi-conductors. It is to be noted, however, that the low resistivity
ceramic materials produced according to the invention do not need to be
maintained in an oxygen-free atmosphere at room temperature in order for
the low resistance properties to be maintained.
DETAILED DESCRIPTION OF THE INVENTION
The production of a highly conductive ceramic material according to the
invention will now be described in the following Examples.
EXAMPLE 1
A ceramic material based on copper and tin oxides was prepared by mixing
cuprous oxide (CuO) and stannic oxide (SnO.sub.2) in a molar ratio
CuO:SnO.sub.2 of 2:1.
The mixed oxides were ball-milled for 16 hours using a Syalon milling
medium and the comminuted oxides were then filtered and dried.
The comminuted oxides were then compacted into the form of a green pellet
at a pressure of 2000 kg/cm.sup.2. The resulting pellet was sintered in
air at 1050.degree. C. for 12 hours to form a solid sintered body.
Preliminary experiments revealed that sintering at only 950.degree. C.
resulted in an incompletely sintered ceramic material which was of a
greyish colour throughout. Sintering at 1050.degree. C. resulted in the
formation of a well sintered body with a greyish outer surface. The
interior of the sintered body was of a reddish shade, believed to be due
to the presence of Cu.sup.1+ ion species. Typically, the resulting ceramic
material had a high resistivity of around 500 .OMEGA..cm.
The ceramic starting material so formed was then annealed by heating at a
temperature of 350.degree. C. for 16 hours in an atmosphere consisting of
40 vol. % hydrogen and 60 vol. % nitrogen.
After cooling, the resulting ceramic pellet had a resistivity of only
3.times.10.sup.-5 .OMEGA..cm (as measured using a 4 probe measuring
technique).
Samples of the annealed and reduced ceramic pellets were sectioned and it
was noted that the reddish interior had disappeared and the pellets were
of a uniform light grey colour throughout. Furthermore, both the interior
and surface regions of the annealed ceramic pellet had a uniform low
resistivity of about 3.times.10.sup.-5 .OMEGA..cm.
EXAMPLE 2
In a further trial, equimolar quantities of tin oxide and potassium
hydroxide were heated together, in the absence of water, at 300.degree. C.
The resulting potassium stannate (K.sub.2 SnO.sub.3) was cooled, dissolved
in water and filtered.
The solution was mixed with an equimolar quantity of aqueous cupric
sulphate and the resulting precipitate collected, washed and dried. X-ray
diffraction showed it to be amorphous or microcrystalline, with no
distinctive peaks indicative of a pronounced crystal structure.
The powder was compressed and heated to 1200.degree. C., the product turned
a reddish colour and X-ray diffraction showed a change in crystalline
state to have occurred.
The fired pellet was then heated at 350.degree. C. for eight hours in a
60/40 v/v stream of H.sub.2 /N.sub.2. The colour changed to a dark grey
shade and on cooling the material was found to have a resistivity
(measured by a 4-probe technique) of about 10.sup.-4 .OMEGA..cm.
EXAMPLE 3
A ceramic material based on zinc and tin oxides was prepared in Example 1.
After being compacted to form a green pellet, the comminuted oxides were
fired in air at about 1280.degree. C., which resulted in a sintered body
with white colour.
The resulting sintered body (which had a high electrical resistance) was
then annealed in a H.sub.2 /N.sub.2 atmosphere at 450.degree. C. for 12
hr. The resulting pellet had resistivity of less than about 10.sup.-3
.OMEGA..cm.
Samples of both the fired and annealed ceramic were mixed with powdered
silver, compacted and fired again at about 1000.degree. C. for 10 hrs,
either in air or in an N.sub.2 atmosphere. The resulting composites showed
extremely high electrical conductivity and once incorporated in a
thermostat as a contact element, showed excellent properties.
The highly conductive ceramic materials which may be produced in accordance
with the invention are of particular use in the manufacture of electrodes
for rechargeable (secondary) electrochemical cells. Such use takes
advantage not only of the high conductivity of the ceramic materials of
the invention, but in view of the valence changes which the metal ion
components are capable of undergoing, electrochemical energy may be stored
in the ceramic material by increasing the proportion of lower valence
state ion species by connecting the cell to an external power supply with
the ceramic electrode connected to the anode. Charged electrochemical
cells formed using conductive ceramic materials according to the invention
have been found to have a particularly highly stable open circuit terminal
voltage. Further, the ceramic material is corrosion resistant.
A particular advantage of the use of conductive ceramic materials of the
invention in the manufacture of battery electrodes is that it eliminates
the need to form conductive elements of graphite and/or carbon black and
the inherent strength of the material can eliminate the need to include a
supporting metal base for the electrode or a contact cap.
By utilising conductive ceramic materials according to the invention in
porous form, the effect of surface area of the electrode can be increased,
thus increasing the area of contact with electrolyte.
A typical electrochemical cell utilising a conductive ceramic material
according to the invention was set up using a cathode formed of Cu.sub.2
SnO.sub.4 produced by the procedure described in Example 1, an anode
formed of nickel coated with hydrated nickel oxide (NiOOH) and a 1M NaOH
electrolyte.
The cell was charged by connecting it to a 2 volt power supply following
which a stable open circuit voltage of 0.96 V was obtained. The open
circuit voltage dropped to 0.89 V after 2 weeks when the test was
terminated owing to evaporation of electrolyte.
The highly conductive ceramic material of the invention in powder form is
particularly useful in the production of conductive materials which
hitherto have relied upon the use of carbon black or metal powder as a
conductive filler.
The conductive ceramic materials according to the invention are less
expensive than equivalent precious metal fillers and they can also be
manufactured to give a smaller particle size than is possible with metals.
Further, the ceramic materials do not oxidise, thus avoiding a
disadvantage of known non-precious metal fillers. Example of specific
applications include the manufacture of conductive adhesives,
electrostatic screening materials and polymer based PTC (positive
temperature co-efficient) devices.
A thermally and electrically conductive adhesive was produced from a
conductive ceramic material according to the invention by dissolving
polystyrene in an organic solvent and adding the conductive ceramic powder
to the resulting solution. After evaporation of the solvent, the resulting
polymer was both thermally and electrically conductive and adhered well to
both metallic and non-metallic substrates. Adhesives produced according to
the invention have advantages over known electrically and thermally
conductive adhesives as a result of the relatively low expense of the raw
materials, the possibility of producing the conductive ceramic material
with a small particle size and the resistance of the ceramic material to
oxidation.
Conductive ceramic materials according to the invention are also
particularly useful in the manufacture of electrical contacts. By virtue
of their resistance to oxidation, they are capable of extending the
service life of contact systems in, for example, switch gear and avoid the
capacitance problem associated with known contact systems resulting from
the creation of surface oxide films on metal surfaces, This in turn
reduces arcing and hence reduces contact erosion. Erosion may be further
avoided even if arcing does occur because of the relatively high melting
point of the ceramic material. Radio frequency emissions are also reduced
in view of the reduced tendency to arcing and the contact surfaces have
improved dimensional stability. Additionally, it has been found that the
fluids used to reduce arcing in conventional contact systems and which are
often corrosive to metals do not adversely affect the novel ceramic
materials of the invention.
Additional applications of conductive ceramic materials according to the
invention include contact electrodes for photovoltaic cells, in which case
it is particularly advantageous for the ceramic material to be applied in
a thin transparent layer. Humidity sensors may also be produced from
CuTiO.sub.3 or SrCeO.sub.3 based ceramic materials dopped with Y or Yb.
By introducing PbSnO.sub.3 into a barium titanate ceramic material,
conductive ceramic materials may be produced according to the invention
which additionally exhibit a piezoelectric effect. By utilising
appropriate metal oxides, ferroelectric ceramic materials may also be
produced.
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